Amplifier stages for current conveyor circuits and current mode feedback operational amplifiers have a number of features in common since current conveyors may be considered as a topological subset of current mode feedback amplifiers. For example, both circuits use current mirrors to generate current signals at a node in response to current signals at an inverting input. For current conveyor circuits, the current signal at the node is the circuit output, and for an ideal current conveyor, the output current is identical to the input current. For current mode operational amplifiers, the current from the high impedance output of the current mirror is applied to an output buffer, and a portion of it is fed back to the inverting input. In both circuits, ideal circuit behavior is approached only where the input resistance of the inverting input is minimized.
In conventional amplifier stages for current conveyors and current mode feedback operational amplifiers, the inverting input is formed by the common emitters of pair of transistors having complimentary conductivity types. The input resistance of the inverting input is the parallel resistance of the emitters of the complimentary transistors. The resistance of each transistor is just the intrinsic emitter resistance, which is approximately equal to 26 ohms per 1 mA of emitter current. For both current conveyor circuits and current mode operational amplifiers, this non-zero input resistance causes the input to deviate from virtual ground, and for current feedback operational amplifiers, it leads to a gain dependence in the closed loop bandwidth, the closed loop transfer function, and the DC closed loop gain.
One solution to the non-zero input resistance of current conveyor and current feedback amplifier circuits is to operate the amplifier stage at high quiescent currents to reduce the input impedance, R.sub.in. However, this approach increases the quiescent power dissipation of the circuit. Further, in order to preserve the unity current gain of current conveyor circuits, any method used to reduce R.sub.in must not introduce differences between the current at the non-inverting input and the current at the high impedance output node.